1. Field of the Invention
The present invention relates to voltage controlled oscillators, and more particularly to voltage controlled oscillators used in local signal generators.
2. Description of Related Art
In radio device, communication is performed by modulating or demodulating data signal. Therefore the radio device has a local oscillator to modulate or demodulate data signal. A voltage controlled oscillator for determining oscillation frequency based on an input voltage is used as the local oscillator. With such a voltage controlled oscillator, there is the case that a phase noise is superimposed on the oscillation output signal. The phase noise is caused by an element characteristic of an element which constitutes the voltage controlled oscillator, noises of a power supply and so on. Since the phase noise is a noise for a receiving signal, it causes degradation of sensitivity or signal quality. The voltage controlled oscillator for eliminating the phase noise is disclosed in Japanese Unexamined Patent Application Publication No. 2001-313527 (Ito) and Japanese Unexamined Patent Application Publication No. 2007-28613 (Ri).
FIG. 7 shows a voltage controlled oscillator of Ito. According to the voltage controlled oscillator circuit of FIG. 7, capacitance values of variable capacitors CV11, CV12 are controlled by a frequency control voltage Vtu. Therefore, oscillation frequency f0 which is generally represented by f0=1/{2π(L×C)0.5} is changed with changing a capacitance component of an LC resonance circuit.
According to the art of Ito, amount of a current which is supplied to an amplifier 143 is changed based on the voltage Vtu which controls the oscillation frequency. This change of the amount of the current changes the gain of the amplifier 143 and the phase noise is lowered. Ri discloses the voltage controlled oscillator which can control amplitudes of oscillation signal by changing gain of an active load.
However, there is a case in the above art that the amplifier amplifies the current excessively by variability of an element which constitutes the voltage controlled oscillator. In this case, although the amplitude is controlled based on a control voltage which sets an outputting frequency as disclosed by Ito, there is a case that the amplifier the current amplifies excessively and the phase noise characteristic deteriorates.